The invention relates to a transducer supply for supplying a transducer with electrical energy from a DC voltage source via a two-wire connection via which the measured value sensed by the transducer is transmitted by a direct current variable between two limit values, DC decoupling in the connection between the transducer and the DC voltage source being achieved by inserting a transformer, the primary winding of which is connected to the DC voltage source via a chopper and the secondary winding of which is connected to a rectifier circuit furnishing at its output connections a direct current generated by rectification of the chopped current transmitted via the transformer in a quantity as dictated by the transducer.
A transducer supply of this kind is intended to supply a passive transducer arranged in an explosion-hazard zone via a two-wire connection with electrical energy whilst simultaneously permitting transmission of the measurement signal furnished by the passive transducer in the opposite direction in the form of a current signal variable between two limit values. In accordance with a popular standard the current signal is variable between 4 mA and 20 mA. A passive transducer contains no electrical voltage source of its own, it instead obtaining the energy needed for its operation via the two-wire connection from a DC voltage source located remote therefrom, it forming the measurement signal by it obtaining from the DC voltage source, in addition to the supply current, a supplementary current dimensioned so that the total current obtained from the DC voltage source corresponds to the transmitted current signal in the range of the two alarm values of, for example, 4 to 20 mA. In addition, communication signals in the form of a pulsed variations may also be impressed on this current signal, as a result of which digital data may be transmitted in both directions. Since the total current may be transmitted in one direction only, namely from the voltage source to the transducer, providing a DC decoupling between the voltage source and the transducer through a transformer is possible by chopping the total current obtained from the DC voltage source at the primary side of the transformer according to the principle of a DC voltage converter and rectifying it at the secondary side of the transformer. Such a means of DC decoupling is a particularly advantageous means of protecting transducers located in an explosion-hazard zone. Providing DC decoupling by means of the transformer of a DC voltage converter permits transmission not only of the DC supply and the DC signal representing the measured value but also the bidirectional transmission of communication signals in the form of pulsed variations impressed on the total current on the condition that the chopper frequency is substantially higher than the frequency of the communication signals.
There is, however, the problem with a transducer supply of the aforementioned kind that it is not possible to connect instead of a passive transducer an active transducer. An active transducer is distinguished from a passive transducer by it being equipped with its own electrical energy supply and it generating the measurement signal in the form of DC signals varying between two alarm values from its own energy supply and outputting it at its outputs. It is not possible to transmit the DC signal furnished by the active transducer in the direction opposite the direction of transmittance of the DC voltage converter.